The technical field of this invention is laser surgery in which a laser is used to ablate biological tissue or otherwise treat regions of the body by irradiation and, in particular, is directed to systems and methods for precisely aligning and confining laser beam exposure to a defined target region during such surgery.
It is known to employ laser sources to erode, ablate, coagulate, alter or otherwise treat surfaces of biological materials. Such laser apparatus is in general relatively complex and demands highly skilled use. For example, laser ablative techniques have been proposed to modify the shape of sensitive surfaces, such as the cornea of the eye to correct vision defects. Extreme care must be taken to confine the ablative procedures to the upper layers of the cornea and to avoid damage to the basement membrane and the posterior endothelial lining of the cornea in such operations.
The use of a laser beam as a surgical tool for cutting incisions, a so-called laser scalpel, has been known for some time (see, for example, U.S. Pat. No. 3,769,963 issued to Goldman et al.). Lasers have also been employed for removal of skin pigmentation abnormalities, "birthmarks," scars, tattoos and the like. Furthermore, lasers have been used for photocoagulation of blood vessels, fusion of biological tissue and selective ablation of delicate biological structures, including the reprofiling or reshaping of the cornea of the eye to correct refractive errors in vision.
A technique for corneal reshaping, involving the use of a laser photoablation apparatus, is known in which the size of the area on the surface to which the pulses of laser energy are applied is varied to control the reprofiling operation. In one preferred embodiment, a beam-shaping shaping stop or window is moved axially along the beam to increase or decrease the region of cornea on which the laser radiation is incident. By progressively varying the size of the exposed region, a desired photoablation profile is established in the surface. For further details on this technique, see also Marshall et al., "Photo-ablative Reprofiling of the Cornea Using an Excimer Laser: Photorefractive Keratectomy," Vol. 1, Lasers in Ophthalmology, pp. 21-48 (1986), and U.S. Pat. No. 4,941,093 issued to Marshall et al., both of which are herein incorporated by reference.
Another approach involves the use of a graded intensity or photodecomposable mask which varies the laser transmission to the target surface, thereby inducing variable ablative depths on the surface. For example, U.S. Pat. No. 4,856,513 entitled "Laser Reprofiling Systems And Methods" which describes methodology for selectively eroding the cornea through the use of an erodable mask. The mask absorbs the surface laser radiation in varying amounts across the corneal surface to provide the desired surface profiles.
One problem of particular noteworthiness in laser corneal surgery and the like is the need for precise alignment of the laser and the target region. Even slight movements of the target can create problems insofar as the reprofiling operations are typically dependent upon the cumulative effects of a number of precisely aligned, discrete ablation steps. Moreover, in some procedures, the problem resides not only in precise positioning of the laser with respect to the eye or other target, but also in precise positioning of intermediate optical components, such as, for example, alignment and angular orientation of a beam-shaping mask or aperture. While gross eye movements can be prevented through the use of a eye restraining cup or the like, the problem of minor movements remains.
Various techniques have been described for tracking eye movements. However, these techniques are usually based on computer tracking or modeling of the eye, coupled with pattern recognition algorithms which attempt to detect, and/or compensate for, eye movements in real time. Such approaches have proved difficult to implement. Even when the eye movements can be monitored in real time, the hardware necessary to steer a laser beam in synchrony with such movements is, likewise, technologically complex and proned to errors. There exist a need for better techniques for tracking eye movements and for precisely aligning and confining laser beam exposure to the target region of the eye during laser surgery.
It is, therefore, an object of the present invention to address the problem of eye tracking, such that compensation can be provided for slight involuntary or inadvertent motions during ophthalmic surgery. More generally, it is an object of the invention to provide better and more reliable tracking mechanisms for laser surgical systems of various types whenever precise alignment with a target is necessary or desirable.